[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Sifaka and Chukar Behavior: Trees Down or Ground Up?



At 12:47 AM 23/01/03 -0800, Jaime A. Headden wrote:
What this appears to be, rather, is that one group of small, predatory
theropods in the trees became adapted to gliding independantly of all
other  avian-style theropods, and that in no way do these finds support
either the trees down or the ground up hypotheses. Merely an interesting
sideline. The tetrapteryx of Beebe and the feathered legs of Feduccia et
al. are interesting predictors of four-winged animals, but again do not
show how flapping flight would have evolved.

In fact I wonder, given the range of new discoveries, whether ANY single fossil discovery can now shed light on this question. What we seem to have is a radiation of feathered or proto-feathered dinosaurs that may well have lived a variety of lifestyles in a variety of habitats, with their feathers possibly serving a variety of functions. It may be that the adaptive shifts involved occurred rapidly and, evolutionarily speaking, fairly easily. Therefore a finding that one species (or another) seems to have been arboreal (or not) may do little more than attest to the variety of lifestyles and behaviours taken up by feathered dinosaurs.


Certainly within living birds there are numerous examples of closely-related species with quite different lifestyles (I have mentioned the flickers (Colaptes) before, which include purely terrestrial species confined to treeless areas and highly-arboreal forest-dwellers, with intermediates).

The assumption of this whole debate seems to be that there was a very clear dichotomy between terrestrial and arboreal modes of life, and that only one of these led to the evolution of flight in living birds. I do not see why this has to be correct - especially if flight or near-flight was being achieved repeatedly in a variety of forms. What, for example, does it mean to be arboreal? There are many birds today that are both arboreal and terrestrial; birds (like herons) that are primarily terrestrial but nest in trees; others (like some wood warblers) that forage in trees but nest on the ground; others (like some grouse) that feed and nest on the ground but will fly into trees to escape predators or to roost.

Why can we not speculate (especially considering Dial's work) that pre-flight adaptations may have evolved neither for a purely arboreal or a purely terrestrial animal? Is it possible that their chief advantage may have been to allow their bearers the versatility to make use of both trees and the ground to varying degrees, for a variety of purposes, rather than to enhance one way of life or the other?


--
Ronald I. Orenstein Phone: (905) 820-7886
International Wildlife Coalition Fax/Modem: (905) 569-0116
1825 Shady Creek Court
Mississauga, Ontario, Canada L5L 3W2 mailto:ornstn@rogers.com